I have a 87 cutlass supreme. The 307 is gettin tired but the car is in great shape other than that. 103,000 miles. I have been looking at crate motors.What will fit, without major mods and what is the best choice. I dont want to hurt originality down the road if I decide to sell. Of course I will keep the original motor. I have all other original wheels,etc. that I have taken off. Many opinions welcome.

 

If you want to eat up the drag strip go with a 455. A 455 will bolt in without much trouble.
But if I where building a driver I would build a nice strong 350 or 403. The blocks are the same as the 307 and the overdrive trans will handle the torque (if it is built right). And if you live in an area that requires emissions testing all of the smog gear can be put back on the car. Install a good low restriction cat and a good exhaust. Use the original HEI Dist and ECM. Use a good carb and a nice dual snorkel air cleaner from a late model 442 and you will have a nice all round car that will eat a Mustang for lunch.
Some will disagree with me but that's what makes this hobby fun. The different ways people take to achieve their goals.

 

If you want to keep the originality of the vehicle then there really is no question, buy a replacement motor from GM, if you can find an Olds motor. You may not be able to. I personally agree with Djorour, a nice 350 or 403 would be very good, and would fit. Again, you aren't going to find a crate engine if you go that route though..... Good luck however you do it.